The International Space Station (ISS) is a marvel of human engineering and international cooperation. This orbiting laboratory has been continuously occupied since November 2000, enabling groundbreaking scientific research and pushing the boundaries of our understanding of life in space. However, living and working in the harsh environment of space presents unique challenges and potential hazards. To ensure the safety of the crew and the integrity of the ISS, NASA and its international partners have established a comprehensive set of safety requirements that all hardware, software, and operations must adhere to.
The ISS Safety Requirements Document
The principal source for technical safety requirements for the ISS is the ISS Safety Requirements Document, also known as SSP 51721. This document defines the safety requirements that must be applied to all end items developed for the ISS, including hardware, payloads, visiting vehicles, and crew support items. The requirements in SSP 51721 are intended to protect the general public, public and private property, flight crews, the ISS itself, and other end items from potential hazards.
The scope of SSP 51721 is broad, covering pressurized and unpressurized end items that are transported, transferred, stowed, operated on, or removed from the ISS. The document also applies to any on-orbit reconfigurations or modifications that could create potentially hazardous conditions. The requirements in SSP 51721 supersede those in previous safety documents, such as SSP 51700 and SSP 50021.
Hazard Control and Failure Tolerance
One of the key principles in the ISS safety requirements is the concept of hazard control and failure tolerance. Hazards are controlled using one of two methodologies: design to tolerate failures or design for minimum risk.
Designing to tolerate failures, also known as failure tolerance, is the preferred approach whenever feasible. This means that the end item must be able to tolerate a minimum number of credible failures or operator errors, determined by the hazard severity level, without creating a hazard. For example, critical hazards require single failure tolerance, meaning that no single failure or operator error can result in a critical hazard. Catastrophic hazards require two-failure tolerance, meaning that no combination of two failures, two operator errors, or one of each can result in a catastrophic hazard.
When failure tolerance cannot be achieved in a logical manner without making the design overly complex or expensive, the alternative approach is to design for minimum risk. This involves using the safety-related properties and characteristics of the design to reduce the associated risk to an acceptable level. Examples of areas where design for minimum risk is acceptable include structures, glass, pressure vessels, pressurized lines and fittings, pyrotechnic devices, and mechanisms in critical applications.
Safety Requirements for Specific Systems
SSP 51721 contains detailed safety requirements for various systems and components of the ISS and its associated end items. These include:
Structures
Structural safety requirements are in place to prevent structural failure of hardware that could lead to hazards to the crew, the ISS, or other end items. Safety-critical structures, such as those containing pressurized systems or hazardous materials, must meet specific design and fracture control requirements. These requirements cover aspects such as material selection, stress analysis, fatigue life, and damage tolerance.
For example, pressurized structures must be designed to leak-before-burst criteria, meaning that a detectable leak must occur before the structure fails catastrophically. This allows for early detection and mitigation of potential hazards. Additionally, safety-critical structures must undergo rigorous testing and analysis to ensure they can withstand the expected loads and environments throughout their operational life.
Electrical Systems
Electrical safety requirements aim to prevent hazards such as electric shock, molten metal, toxic material release, fire, and touch temperature hazards. These requirements cover aspects such as wiring and circuit protection, connector mate/demate, biomedical instrumentation, batteries, and capacitors.
One key aspect of electrical safety is the prevention of inadvertent contact with energized conductors. This is achieved through proper insulation, guarding, and labeling of electrical components. Additionally, electrical systems must incorporate appropriate circuit protection devices, such as fuses and circuit breakers, to prevent overloads and short circuits that could lead to fire or other hazards.
Batteries, which are essential for many ISS systems and payloads, have their own set of safety requirements. These include provisions for charge control, overcharge protection, and containment of hazardous materials in the event of a battery failure. Lithium-ion batteries, which are commonly used due to their high energy density, require additional safety features such as cell-level protective devices and robust thermal management systems.
Command and Data Handling
Safety requirements for command and data handling systems ensure that hazardous commanding and on-board computer systems do not pose risks to the crew or the ISS. This includes requirements for hazardous commanding, software protections, and data integrity.
Hazardous commanding refers to commands that could potentially lead to hazardous conditions if executed inadvertently or out of sequence. To prevent such occurrences, the ISS safety requirements mandate the use of multiple independent inhibits on hazardous commands. These inhibits can include hardware interlocks, software checks, and procedural controls.
Software safety is another critical aspect of command and data handling systems. Safety-critical software must undergo rigorous development, testing, and verification processes to ensure its reliability and robustness. This includes the use of formal methods, such as requirements traceability and code analysis, to identify and eliminate potential software hazards.
Pressure Systems
Pressure systems on the ISS can be classified as low pressure or high pressure and can contain hazardous or non-hazardous fluids or gases. Safety requirements for pressure systems cover aspects such as sealed containers, pressure vessels, and pressurized lines, fittings, and components.
Pressure vessels, which are used to store gases or liquids under pressure, must be designed, fabricated, and tested in accordance with recognized industry standards such as ASME Boiler and Pressure Vessel Code. These standards ensure that the vessels can safely withstand the expected operating pressures and environments.
Pressurized lines and fittings must be designed to withstand the maximum expected operating pressure (MEOP) with appropriate safety factors. Additionally, these components must be compatible with the fluids or gases they contain and must be protected against damage from external sources such as impacts or thermal loads.
Sealed containers, which are used to store hazardous materials or to provide a controlled environment, must be designed to maintain their integrity under the expected operating conditions. This includes provisions for leak detection, pressure relief, and containment of hazardous materials in the event of a leak or rupture.
Pyrotechnic Systems
Pyrotechnic devices, such as those used for deployment or separation functions, must meet stringent safety requirements to prevent inadvertent activation and ensure proper containment of pyrotechnic products.
One key aspect of pyrotechnic safety is the use of multiple independent inhibits to prevent inadvertent activation. These inhibits can include mechanical safing devices, electrical safing devices, and arm/fire commands. Additionally, pyrotechnic devices must be designed to contain all pyrotechnic products, such as gases and debris, to prevent damage to surrounding hardware or injury to crew members.
Pyrotechnic devices must also undergo rigorous testing and qualification to ensure their reliability and performance under the expected operating conditions. This includes environmental testing, such as vibration and thermal cycling, as well as functional testing to verify the proper operation of the device.
Extravehicular Activity (EVA)
Safety requirements for EVA aim to protect crew members during spacewalks, addressing issues such as temperature extremes, sharp edges and protrusions, and entanglement hazards.
EVA suits, which provide life support and protection for crew members during spacewalks, must meet stringent design and performance requirements. These include provisions for thermal control, radiation protection, and micrometeoroid and orbital debris (MMOD) protection. Additionally, EVA suits must be designed to minimize the risk of entanglement or snagging on ISS structures or equipment.
EVA tools and equipment must also be designed with crew safety in mind. This includes the use of tethers to prevent loss of tools, as well as the incorporation of safety features such as guards and interlocks to prevent inadvertent activation or release of hazardous materials.
Verification and Safety Reviews
To ensure compliance with the ISS safety requirements, end item providers must verify the implementation of the requirements through a combination of analysis, testing, demonstration, and inspection. The ISS Safety Review Panel (ISRP) is responsible for assessing end item compliance with the safety requirements and determining the acceptability of verification methods.
The safety review process, as defined in SSP 30599, involves a series of phased safety reviews where the ISRP assesses the safety hazards related to the design, operations, and functional capabilities of ISS end items and associated ground support equipment. End item providers must submit hazard analysis data and other safety verification deliverables at each phase of the safety review process.
The phased safety review process typically includes the following milestones:
- Phase 0: Initial safety review, where the end item provider presents the preliminary design and identifies potential hazards and safety requirements.
- Phase I: Intermediate safety review, where the end item provider presents the detailed design and the results of initial hazard analyses and safety verification activities.
- Phase II: Final safety review, where the end item provider presents the final design and the results of all hazard analyses and safety verification activities.
- Phase III: Safety certification review, where the ISRP assesses the end item’s compliance with all applicable safety requirements and grants safety certification for on-orbit operations.
Throughout the safety review process, the ISRP works closely with the end item provider to identify and resolve any safety issues or concerns. This collaborative approach ensures that all ISS end items meet the highest standards of safety and reliability.
Continuous Improvement of ISS Safety Processes
The ISS program is committed to continuous improvement of its safety processes and tools to ensure the ongoing safety of the crew and the station. This includes regular reviews and updates of safety requirements documents, as well as the implementation of new technologies and best practices in hazard analysis and risk management.
One example of this continuous improvement is the recent upgrade of the ISS hazard data management system. The new web-based system, called the Mission Assurance System (MAS), provides improved accessibility, searchability, and integration of safety and hazard data across the ISS program. This system streamlines the hazard reporting and approval process, reducing workload and increasing compliance with safety requirements.
Another area of focus for continuous improvement is the incorporation of lessons learned from past safety incidents and near-misses. The ISS program maintains a comprehensive database of safety-related events and anomalies, which is used to identify trends and develop corrective actions to prevent recurrence. This data-driven approach to safety management ensures that the ISS program remains proactive in identifying and mitigating potential hazards.
Summary
The ISS Safety Requirements Document, SSP 51721, is a comprehensive set of technical requirements designed to ensure the safety of the crew, the ISS, and associated end items. By adhering to these requirements and undergoing rigorous safety reviews, NASA and its international partners can continue to operate the ISS safely and successfully, enabling groundbreaking scientific research and paving the way for future human exploration of space.
The ISS safety requirements cover a wide range of systems and components, including structures, electrical systems, command and data handling, pressure systems, pyrotechnic systems, and EVA. These requirements are based on the principles of hazard control and failure tolerance, with the goal of minimizing the risk of hazardous conditions that could threaten the crew or the station.
Compliance with the ISS safety requirements is ensured through a rigorous verification and safety review process, which involves close collaboration between end item providers and the ISS Safety Review Panel. This process ensures that all ISS end items meet the highest standards of safety and reliability before being approved for on-orbit operations.
Looking to the future, the ISS program remains committed to continuous improvement of its safety processes and tools. By leveraging new technologies, incorporating lessons learned, and maintaining a proactive approach to hazard identification and mitigation, NASA and its international partners can continue to ensure the safety of the crew and the success of the ISS mission for years to come.
